Plants perennial, sometimes with rhizomes, forming tufts,
small to large clumps, or colonies. Flowering stems erect to less commonly
ascending or arched, glabrous. Leaf sheaths glabrous or the lowermost roughened
or hairy, sometimes glaucous, the ligule short, membranous, rarely absent. Leaf
blades flat or with the margins inrolled, glabrous or roughened to hairy,
sometimes with a pair of small auricles at the base. Inflorescences erect or
arched to nodding, usually with the base held above the uppermost leaf sheath
(except sometimes in E. virginicus), with numerous erect to ascending or
spreading spikelets spaced densely or loosely along the axis, the internodes
3–25 mm long in various species. Spikelets single or in clusters of 2(3 or 4)
at each node of the inflorescence, all similar in size and appearance, all
fertile, linear in outline, with 2–8 florets, disarticulating above the glumes
or sometimes also at the spikelet base (disarticulating at the nodes of the
inflorescence axis in E. longifolius, the joints shed as a unit with the
attached spikelets). Glumes present and similar in size and appearance, less
commonly highly reduced and uneven or absent, elliptic‑lanceolate to
linear, sometimes thickened and hard at the base, the tip with a long or short
awn (except sometimes in E. virginicus), glabrous or roughened to hairy.
Lemmas with the tip pointed or awned, rounded on the back, glabrous or roughened
to hairy, lacking stiff, spinelike hairs along the keel and the margins. Paleas
rounded to bluntly pointed at the tip (except in E. canadensis, with
pointed to often cleft tips), as long as or shorter than the body of the
lemmas. Fruits 4–6 mm long, linear‑oblong, usually somewhat flattened and
grooved, dark brown to reddish brown. About 300 species, nearly worldwide.

The generic taxonomy of Elymus is confusing and
controversial. Many of the species with spikelets occurring singly along the
axis were placed in Agropyron in the older literature (Steyermark,
1963), but that genus has become restricted to a small group of closely related
Old World species (see treatment above). The genera Hystrix Moench and Sitanion
Raf. were formerly recognized as distinct, based on minor innovations of
spikelet morphology and placement, but generally are now accepted as sections
within Elymus (even in the strict sense). On the other hand, the
relatively broad circumscription of Elymus accepted here includes
several large species groups that have been segregated into such genera as Elytrigia
Desv., Leymus Hochst., Pascopyrum Á. Löve, Pseudoregneria
(Nevski) Á. Löve, and Thinopyrum Á. Löve, among others. These genera
generally are based on cytogenetic analysis of genome types and breeding
systems, and some are more easily circumscribed morphologically than others. A
number of authors (Estes and Tyrl, 1982; Arnow, 1987) have expressed doubts as
to the utility of breaking Elymus apart into these more narrowly defined
groups. Recently, Assadi and Runemark (1995) studied chromosomal homology in
artificial hybrids of various Old World taxa and concluded that although Agropyron
(as treated in the present work) has a genome that is well differentiated from
that of Elymus, other groups, including Elytrigia, Pseudoregneria,
and Thinopyrum, should continue to be treated as part of a broadly
circumscribed Elymus.

Hybridization between Elymus species is relatively
common, as well as hybridization between Elymus and related genera, such
as Hordeum. However, few such hybrids from Missouri are represented in
herbaria. Collectors who find plants with unusual morphology should note other
species of Triticeae that may be growing in the vicinity.

Inflorescences with the axis persistent, not breaking into segments at maturity, the spikelets disarticulating above the glumes or sometimes also at the spikelet base; glumes (including the awn) 11–40 mm long or sometimes lacking

Inflorescences usually curved or nodding at maturity; glumes 0.2–1.0 mm wide, not or only slightly expanded above the relatively straight base, persistent on the inflorescence axis and not shed with the florets